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I have a 2002 cr 250. The cylinder is oem with about 60hrs on it. OEM pistons are discontinued. Im a machinest with extensive experiance with measuring devices (mics of all sorts, bore guage, etc) but I am not a pro engine builder. My cylinder measures 2.614 as checked with a dial bore guage calibrated with my ISO 9000 certified OD mics. I ordered a vertex piston (size B, the only size available) that is marked 66.35 = 2.6122 allowing almost .002 clearance, but it actually mics 2.613. Vertex reccomends .0019-.0031 clearance. That would only give me about .001 clearance (half of what they recommend). So I ordered a weisco pro lite. It says that clearance should be .0015 but it also mics right at 2.613 which is also only about .001 clearance. As I understand the forged weisco should allow more clearance for thermal expansion. Why then is their recommended clearance less than the cast vertex? If these are my 2 choices and both are running about .001 clearance which should I put in? The weisco forged is only .0005 tighter than they recommend but seems DANGEROUSLY tight especially for a forged. Or should I put in the cast vertex that is .001 tighter than recommended but probably safer than the forged. Pretty frustrated that neither of these pistons seems to offer the proper clearance. I dont want to (or feel I should have to) send my oem jug out for bore/recoat of only .001 bigger to fit pistons that are made to sloppy tolerances. Also as a second question the weisco metal is in the white but the vertex has a black coating on it. Could I lightly polish about .001 off the OD of the vertex piston? It would remove that black coating but since the weisco dosnt have the same coating would that really be a problem? Or maybe I could polish some material off the weisco to get a little more clearance. Most people seem like they just order a piston and drop it in without even measuring but Im kinda anal about this stuff and feel like I have a problem. Anyone with vast experiance building motors and measuring pistons and cylinders have any advice or thoughts for me? Im not sure what to do.
More than one place?
Paw Paw
Or did you actually use s feeler gauge on the assembly?
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A 100 years ago when I would run into a tight fitting new forged piston in a good bore I would do the following to insure a good fit and long life.
1. Assemble the engine.
2. Do a couple of heat cycles under no load.
3. Pull the cylinder and piston and review for any markings.
4. Sand the piston skirts to a very high gloss and then reassemble.
5. Repeat the heat cycles and repeat the piston inspection and sanding.
Remember that a forged piston will expand at a faster rate than the cylinder so a complete warm up of the engine under no load is very important for each ride. You can also run a tighter fit with the forged piston than a cast unit if you do the proper no load warm up each ride.
Paw Paw
When hot, both pistons will have same clearance in the bore.
Also do NOT try and resize the piston yourself. A piston has cam and barrel cut into the skirts and without the proper skirt turn machine you will ruin the part.
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Pit Row
I use bore gauges and mikes on a daily basis.
Just use slim width verified shim stock or feeler guage at the belly of the piston for checking. Go/no go
It is a direct, first principle; superior to the multiple measurements, translations and calculations the alternatives involve.
Pro X offers 53.94, 53.95, 53.96 and 53.97mm for the 93 CR125. Is their 53.94mm piston still too big?
Pro X offers 66.34, 66.35, 66.36 and 66.37mm pistons for the 02 CR250.
Pro X part number 01.1323.A is their 66.34 piston and should work for you, but these have coated skirts.
The newer 05-07 pistons from Pro X are not coated but they are dome top designs so you can not interchange with 02-04 without also swapping heads.
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